Large void-free polyethylene castings comprising high density polyethylene in a low density polyethylene matrix

ABSTRACT

Large void-free castings of polyethylene can be made by blending high density polyethylene pellets with low density polyethylene powder, followed by heating to above the melting point of the low density polyethylene and below, or only slightly above, the melting point of the high density polyethylene. Flame proofing additives, crosslinking agents and other materials can be readily incorporated into the blend before fusing. The mixture of high density and low density polyethylene significantly reduces the amount of shrinkage which takes place during the cooling from the melting temperature to ambient temperature. The low density polyethylene should have a sufficiently low melt viscosity so that it will flow under small pressures above the melting temperature.

United States Patent Salyer et al.

LARGE VOID-FREE POLYETHYLENE CASTINGS COMPRISING HIGH DENSITYPOLYETHYLENE IN A LOW DENSITY POLYETIIYLENE MATRIX Inventors: Ival O.Salyer, Dayton; Charles J.

North, West Carrollton, both of Ohio Assignee: Monsanto ResearchCorporation, St.

Louis, Mo.

Filed: Dec. 13, 1973 Appl. No.: 425,894

Related US. Application Data Continuation of Ser. No. 219,498.

US. Cl 260/897 A; 260/42.39; 260/42.46; 264/331 Int. Cl. C08L 23/06Field of Search. 260/897 A, 94.96 A, DIG. 32; 161/252 References CitedUNITED STATES PATENTS 1/1960 Cole 18/59 3,231,636 1/1966 Snyder et a1260/897 3,474,051 10/1969 Chappelear et al... 260/2.5 3,478,132 11/1969Randolph 260/897 X 3,511,742 5/1970 Rasmussen 260/897 X PrimaryExaminer-Murray Tillman Assistant ExaminerC. J. Seccuro Attorney, Agent,or FirmBruce Stevens ABSTRACT Large void-free castings of polyethylenecan be made by blending high density polyethylene pellets with lowdensity polyethylene powder, followed by heating to above the meltingpoint of the low density polyethylene and below, or only slightly above,the melting point of the high density polyethylene. Flame proofingadditives, crosslinking agents and other materials can be readilyincorporated into the blend before fusing. The mixture of high densityand low density polyethylene significantly reduces the amount ofshrinkage which takes place during the cooling from the meltingtemperature to ambient temperature. The low density polyethylene shouldhave a sufficiently low melt viscosity so that it will flow under smallpressures above the melting temperature.

2 Claims, No Drawings LARGE VOID-FREE POLYETI-IYLENE CASTINGS COMPRISINGHIGH DENSITY POLYETHYLENE IN A LOW DENSITY POLYETHYLENE MATRIX This is acontinuation of application Ser. No. 219,498, filed Jan. 20, 1972, nowabandoned.

BACKGROUND OF THE INVENTION This invention relates to polyethylenecastings and more particularily to a composite of high densitypolyethylene embedded in low density polyethylene and the method ofmaking such castings.

Polyethylene has become an important commercial product. It is producedfrom ethylene monomer, a gas derived from natural gas or from thecracking of crude oil. A high pressure reaction with trace amounts ofoxygen as a catalyst produces high pressure or low density polyethylene.This material is also known as conventional polyethylene.

Other polyethylene polymers, known as low pressure" or high density"polyethylene, are well known in the art. These ethylene polymers havevery different properties from the low density polyethylene, and areprepared by reacting ethylene monomer in the presence of a metallicoxide catalyst. Polymers so produced may be linear, or may containcontrolled amounts of branching. Nominal densities of high densitypolyethylene are from 0.939 to 0.965 gram per cc.

The American Society for Testing and Materials types polyethylene asfollows: Type 1 is low density polyethylene having a mass per unitvolume ranging from 0.910 to 0.925 gram percc; Type 2 is medium densitywith a density ranging from 0.926 to 0.940 gram per cc; and Type 3 ishigh density polyethylene with a density ranging from 0.941 to 0.960gram per cc.

High density polyethylene having a weight average molecular weight ofabove 1.5 million, as determined by viscosity measurements, is calledultrahigh molecular weight polyethylene. Essentially linear instructure, it has some properties superior to those of linear highdensity polyethylene of lower molecular weight.

A number of blends of these polyethylenes have been made to achieveparticular properties and are well known in the prior art. For example,U.S. Pat. No. 3,194,850 teaches a homogeneous blend of two or morepolyethylene resins of different molecular weight by forming a melt ofthe resins, cooling the resins to form a solid, and reducing the solidblend to a finally divided state and remelting. Simularly, U.S. Pat. No.3,592,881 teaches the preparation of a heat-scalable film prepared bycrosslinking an oriented film of a blend of low density and high densitypolyethylenes for packaging. The latter patent is directed towardproviding a heat shrinkable polyethylene film having a broad heatsealing range which is suitable for use on automatic packagingequipment.

Polyethylene is typically used for wire and cable coatings and can bemolded by a number of techniques such as thermoforming, rotationalmolding, injection molding, film extrusion, and blow molding; however,thick castings of polyethylene are difficult to prepare because ofshrinkage which'either causes the polyethylene to pull away from thewalls of the mold during cooling or causes voids in the center of thecasting. The present invention is directed toward a process and acomposition to make large void-free castings of polyethylene.

SUMMARY OF THE INVENTION It is one object of this invention to provide aprocess to cast large void-free castings of polyethylene. It is anotherobject to provide a composition of matter that can be used for largevoid-free castings of polyethylene.

These and other objects are achieved by a composite material whichcomprises 25 to 50 weight percent low density polyethylene and 50 toweight percent high density polyethylene embedded in the low densitypolyethylene. The invention further contemplates a method of making avoid-free molding of polyethylene which comprises the steps of placingin a mold a blend of 25 to 50 weight percent low density polyethylenepowder with 50 to 75 weight percent high density polyethyleneparticulates, heating the blended polyethylene particulates to atemperature above the melting point of the low density polyethylene, butbelow the temperature at which the high density polyethylene will flowcausing the molten low density polyethylene to flow to fill the voids,and thereafter cooling the blended void-free polyethylene composite.

The polyethylene composite can be conveniently cast by mixing a powderand pellets. Ideally, the powder is the lower density polyethylenehaving a size range of about 16 mesh to 50 mesh and the pellets are thehigher density polyethylene having a size range of about onesixteenthinch to one-fourth inch on a side, or even larger. Cubic pellets arepreferred, although spheres and cylinders or platelets can also be used.The mixture of powder and pellets provide a higher apparent density thaneither powder or pellets alone, which is advantageous because itminimizes the shrinkage of the casting during the melting andsolidification. The small particle size powder also provides a largesurface area for better distribution of powder or liquid additives. Thepellets in the blend improve abrading action during the blending cycleand thus help breakdown lumps or agglomerate of any additives that arepresent.

It is desirable that the pellets have a higher melting point than thepowder. Conveniently, the powder may be Type 1 polyethylene while thepellets are Type 2 or 3, or the powder may be Type 2 and the pellets maybe Type 3. Indeed it has been found that ultrahigh density polyethyleneis quite satisfactory. For the process to operate successfully it isessential that the powder used as the matrix binder have a sufficientlylow melt viscosity such that it will flow under small pressures at themelting temperature; for example, the melt index of the low densitypolyethylene powder as determined by A.S.T.M. procedures should probablybe at least 0.1 decigram per minute and preferably 1 to 20 decigrams perminute. The use of low density polyethylene having a melt indexsubstantially higher than about 40 decigrams per minute would result incastings which would become brittle at low temperature and would havepoorer tensile strength and elongation than those of lower melt index.

The melt viscosity of the high density polyethylene is not criticalsince in most cases it will not be heated above its melting point.However, a melt index in the same general range of 0.1 to 20 decigramsper minute as specified for the low density powder is desirable,although a melt index up to 40 decigrams per minute would besatisfactory.

it has been found that the ratio of high density pellets to low densitypowder is critical. A satisfactory composite can be made using 50 to 75percent by weight of high density polyethylene pellets blended with 50to 25 percent by weight of low density polyethylene powder. At less than25 percent polyethylene powder the composite contains voids due to theshrinkage of the low density polyethylene when it is melted. At above 50percent low density polyethylene the high density polyethylene pelletsare not evenly distributed in the composite, causing density gradientsin the completed casting.

The composite of low density and high density polyethylene may containadditives to achieve desired properties. As an example, flame-proofingadditives and crosslinking agents can be readily incorporated into theblend before fusing. Crosslinking can be achieved by incorporating fromabout 0.5 to about 5 weight percent of an organic peroxide crosslinkingagent, such as dicumyl peroxide, in the composition and curing attemperatures of about 175 to about 200C. The crosslinked polyethyleneexhibits the same crystalline melting point as the original resins. Whenthe crosslinked polymer is heated to a temperature above the crystallinemelting point, it is converted from a plastic to a rubbery melt. Oncooling the polymer reverts to a plastic. Crosslinked polyethyleneresins provide a material with enhanced properties, especially creepresistance at high temperatures and stress cracking resistance. Otheradditives such as antioxidants, and in some instances ultraviolet lightstabilizers, can be added to the blend before fusing to achievedesirable characteristics.

It has also been found that a composite of high density polyethylenefibers can be blended with low density polyethylene powder to produce acomposite having superior properties. As an example, when sheets of thiscomposite are prepared by the same techniques as used to prepare acomposite using high density polyethylene particles and low densitypolyethylene powder, the composite containing the fibers aresubstantially more resistance to tear than the low density polyethylenealone.

The void-free composites are prepared by placing in a mold a blend of 25to 50 weight percent low density polyethylene powder with 50 to 75weight percent high density polyethylene pellets or fibers and heatingthe blended polyethylene to a temperature above the melting point of thelow density polyethylene, but below the melting point, or only slightlyabove, say 25C. above, the melting point of the high densitypolyethylene. When only a very slight pressure is applied to the mold,the molten low density polyethylene will flow to fill the voids and thusimbed the high density polyethylene in the low density polyethylene.Thereafter, the casting is cooled to form a void-free polyethylenecomposite.

A particularily advantageous composition contains 33 percent low densitypolyethylene powder and 67 percent high density polyethylene pellets.

Higher pressures can advantageously be used to expedite flow of the lowdensity polyethylene and help to significantly shorten the moldingcycle. However, pressures of several thousand pounds per square inch(p.s.i.), such as typically used in injection or extrusion molding arenot necessary or desirable. Molding pressures of 0-100 p.s.i. and notgreater than 1,000 p.s.i. are contemplated.

An important distinction of the composites of this invention from theprior art products can be observed upon visual examination. During theprocess to prepare these composites, the polyethylene is heated abovethe melting point of the low density polyethylene, and perha'ps'abovethe melting point of the high density polyethylene. Since the moltenpolyethylene is not stirred, the high density polyethylene willcrystallize first upon cooling, and the separate phase structure of thehigh density polyethylene wil be preserved, and is clearly visible.

An excellent radiation shield suitable for shipping neutron sources wasprepared by blending l to 10 weight percent boric acid with 30 to 50parts of low density polyethylene powder and 50 to parts of high densitypolyethylene pellets. The neutrons produced by the reaction of an atomicparticle on a light element, such as boron or beryllium, are reflectedby the hydrogen in the polyethylene and the boron incorporated as anadditive. Thus, neutron sources having an emission of 1 X 10 neutronsper square meter per second can be successfully shipped without exposingthose nearby to radiation by embedding the source in a shield of thepolyethylene composite of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is furtherillustrated by but not limited to the following examples.

EXAMPLE 1 About 21.6 parts of MPE 210 pellets, a high density, blowmolding polyethylene resin having a density of about 0.955 grams percubic centimeter available commercially from Monsanto Company, St.Louis, Miss. was blended with 10.75 parts of Microthene 720 a finelydivided, low density polyethylene available commercially from U.S.Industrial Chemical Company, New York, N.Y., which has a density of0.915, a melt index of 12, and particles less than 40 mesh and 1.77parts of boric acid. The various components were tumble blended forabout 45 minutes and placed in a large steel mold approximately 13inches by 13 inches by 19 inches high. The mold containing the blendedingredients was placed in an oven at C. for 16 hours. A lid was thenplaced on top of the ingredients and the gentle pressure of the lidcaused the molten low density polyethyiene to flow to fill any voids.The mold was heated in an oven at 130C. for another 16 hours. The longheating times were necessary to permit the heat to penetrate thepolyethylene. When the steel mold containing the blended polyethylenewas cooled and the casting removed, a large void-free composite of highdensity polyethylene pellets in a low density polyethylene matrix wasobtained.

EXAMPLE 2 The process of Example 1 was repeated using 21.6 parts of MPE210 and 21.6 parts of Microthene 720 with 2.4 parts of boric acid andheated for 16 hours at C. A large void-free casting was obtained whichwas satisfacotry for use as a nuclear radiation shield forneutron-emitting radiation sources.

EXAMPLE 3 The procedure of Example 1 was repeated except that theblended composition contained about 9806 grams of MPE 210, 4881 grams ofMicrothene 720, 804 grams of boric acid, and 309 grams of Dicup R. Theingredients were placed in the stainless steel mold and heated to 150C.overnight. Samples cut from the large block revealed that the castingwas void-free.

6 cordingly modifications are contemplated which can be made withoutdeparting from the spirit of the described invention.

We claim:

1. A composite material which comprises 50 to weight percent highdensity polyethylene pellets having a size range of about one-sixteenthinch to one-fourth inch in a matrix of 25 to 50 weight percent lowdensity polyethylene.

2. A composite of claim 1 wherein the polyethylene is crosslinked withabout 0.5 to about 5.0 weight percent dicumyl peroxide.

1. A COMPOSITE MATERIAL WHICH COMPRISES 50 TO 75 WEIGHT PERCENT HIGHDENSITY POLYETHYLENE PELLETS HAVING A SIZE RANGE OF ABOUT ONE-SIXTEENTHINCH TO ONE-FOURTH INCH IN A MATRIX OF 25 TO 50 WEIGHT PERCENT LOWDENSITY POLYETHYLENE.
 2. A composite of claim 1 wherein the polyethyleneis crosslinked with about 0.5 to about 5.0 weight percent dicumylperoxide.